Understanding the structure and properties of geological formations can reduce the cost of drilling wells for oil and gas exploration. Measurements made in a borehole (i.e., down hole measurements) are typically performed to attain this understanding, to identify the composition, structure, properties, and distribution of material that surrounds the measurement device down hole. To obtain such measurements, logging tools of the acoustic type are often used to provide information that is directly related to geo-mechanical properties.
Traditional acoustic tools utilize transmitters to create pressure waves inside the borehole fluid, which in turn create several types of waveguide modes in the borehole. These modes can be processed to determine formation properties, such as compression and shear wave velocity of the formation. Traditional tools use electrically excited (i.e., active) sources to produce movement in the formation. To support broad-band operation and overcome noise, transmitters often have a high power requirement. As a result, large capacitor sections are located in the tool, with a significant increase in tool length. Moreover, mechanical and electrical design of the transmitters is usually difficult, increasing the tool manufacturing and maintenance costs considerably.